The present invention relates to delay spread estimation and selection of a receiver based on such estimation, and more particularly to delay spread estimation for a radio frequency signal path and selection of a receiver equalizer based on such estimation. Even more particularly, the present invention relates to delay spread estimation of a cellular telephone signal path, and selection of a receiver equalizer based on such estimation, wherein a delay spread threshold for selection of the receiver equalizer is a function of signal-to-noise ratio.
Communication channels in the cellular environment commonly impose a combination of distorting effects on transmitted signals. Rayleigh fading, where a signal's perceived power level rises and falls rapidly over a wide range, results from the combination (interference) of signals that have traversed paths differing in length by at least a significant fraction of a wavelength (i.e., about 30 cm. for cellular). This type of interference is known as multi-path interference. Differences in path transmission times that approach the time taken to transmit a symbol result in a second problem called delay spread.
Delay spread results in reception of multiple delayed replicas of a transmitted signal. Each Rayleigh faded replica has randomly distributed amplitude and phase, and the rate at which this complex quantity varies is constrained by the Doppler bandwidth associated with a vehicle's speed. In a frequency nonselective environment, the sampled outputs of a receiver's matched filter provides uncorrelated estimates of the transmitted data. As such, in terms of discrete time samples, the channel has exhibited an impulse response proportional to a delta function. With delay spread, on the other hand, the discrete time channel impulse response is extended to introduce energy at a number of symbol times. The effect of the channel on the transmitted signal, in turn, may be viewed as the convolution of the transmitted information with the channel's impulse response. The channel, therefore, emulates a convolutional coding process (encoder).
This leads to the possibility of estimating the transmitted information through the use of methods analogous to typical decoding of convolutional codes, i.e., maximum likelihood sequence estimation techniques. Unlike the more widely applied forward error correction decoding environment, the details of the encoding process in a reverse error correction decoding environment, are not known a priori by the receiver. Issues related to the need to estimate the form of the encoding process are addressed by this invention.
For the North American digital cellular system, a number of documents define the standards of implemented components. With respect to this invention, the following are of interest: "Dual-Mode Mobile Station-Base Station Compatibility Standard" denoted here as IS-54, EIA/TIA Project Number 2398, Rev. A January 1991 and "Recommended Minimum Performance Standards for 800 MHz Dual-Mode Mobile Stations", denoted here as IS-55, EIA/TIA Project Number 2216, April 1991, both of which are incorporated herein by reference.
It is therefore desirable to provide an enhancement to the processing performed by equalizers for use in mobile telephones that provides for system complexity reduction and that provides for better performance in a fading channel.
One example of an enhancement to the process performed by equalizers for use in mobile telephones is shown in U.S. Pat. No. 5,263,026 issued to Parr et al. (referred to hereinafter as the '026 patent), incorporated herein by reference. Problematically however, equalizers such as shown in the '026 patent, while providing significantly enhanced processing of signals under conditions where delay spread has distorted the signals, can significantly degrade the quality of the signals when delay spread is negligible.
Thus, what is needed is a cellular mobile telephone receiver system that includes an equalizer, but that can detect when equalization by the equalizer is not needed and/or is disadvantageous. Such cellular mobile telephone receiver should be able to eliminate (or disable) equalization by the equalizer in response to such determination by deselecting the equalizer when it is not desirable.
The present invention advantageously addresses the above and other needs.